RE: Time taken to Upgrade from SAP EHP 0 to EHP 6
the total time depends on the database size, it can vary from some hours on a test system with no data at all to several days on a high loaded server. It is really very difficult to estimate a total runtime.
I advise you to perform some test upgrades first (of course) on a system copied from your DEV, from your QAS and then from your PRD.
Here is some reading about this recurrent question:
I hope this information helps.